This figure shows a water droplet on a solid surface and the tensioactive forces occuring at each interface: Solid-air, Liquid-solid and Air-liquid.  Note that the point of contact of air, liquid and solid is in equilibrium. Because the tensioactive forces are a function solely of the ratios of cohesive and adhesive forces, the angle of contact will take a value that achieves equilibrium (minimum energy). Thus, at any given temperature, the contact angle depends solely on the materials involved.